1. Technical Field
The present invention relates to a modulation apparatus, a phase setting method, and a test apparatus.
2. Related Art
A conventional LINC circuit is known that outputs a modulated signal, as shown in Non-Patent Documents 1 to 5, for example. The LINC circuit uses two IQ modulators to generate two modulated signals with different phases and fixed amplitudes, and outputs the desired amplitude and phase signals by adding together the two modulated signals generated by the two IQ modulators. This type of LINC circuit can output a signal with a high amplitude and low distortion, regardless of the linearity of the amplifier at the output stage.
Non-Patent Document 1: Lars Sundström, “Spectral Sensitivity of LINC Transmitters to Quadrature Modulator Misalignments”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 49, NO. 4, JULY 2000
Non-Patent Document 2: Fernando J. Casadevall, and Antonio Valdovinos, “Performance Analysis of QAM Modulations Applied to the LINC Transmitter”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 42, NO. 4, NOVEMBER 1993
Non-Patent Document 3: Gwenael Poitau, Ahmed Birafane and Ammar Kouki, “Experimental Characterization of LINC Outphasing Combiners' Efficiency and Linearity”, IEEE Radio and Wireless Conference, 2004
Non-Patent Document 4: Young Yun Woo, Jaehyok Yi, Youngoo Yang, and Bumman Kim, “SDR Transmitter Based on LINC Amplifier with Bias Control”, Microwave Symposium Digest, 2003 IEEE MTT-S International
Non-Patent Document 5: Xuejun Zhang, Lawrence E. Larson, and Peter Asbeck, “Design of Linear RF Outphasing Power Amplifiers”, (USA), Artech House, 2003
In a LINC circuit, the control section calculates two phases based on the phase and amplitude designated from the outside, and sets the two calculated phases in the two IQ modulators. Regardless of which of the two IQ modulators the two calculated phases are set in, the amplitude and phase of the output signal from the LINC circuit are theoretically the same. However, if the frequency bands of the modulated signals output from the IQ modulators are wide, the distortion of the output signal is increased by the hardware.
Furthermore, when the amplitude of the output signal is zero, two modulated signals differing by 180° should be output. However, the frequency band widens if the phases of the two modulated signals are not set appropriately, and this increases the distortion in the output signal due to the hardware.
If the phases for the two IQ modulators are set to achieve band-limiting, the distortion in the modulated signals due to the hardware is decreased. However, the error of the phases set for the two IQ modulators increases. On the other hand, if the phases for the two IQ modulators are not set to achieve band-limiting, the phases of the two IQ modulators can be set in the usual manner but the distortion of the output signal increases.